Nano α-Fe₂O₃ Self-Assembled Hybrid Biofilm Boosts Hydrogen Autotrophic Denitrification in a Three-Dimensional Biofilm Electrode Reactor

The performance of three-dimensional biofilm electrode reactors (3D-BERs) in hydrogen autotrophic denitrification is determined by electroactive biofilms. Herein, a self-assembled hybrid biofilm (SAHB) is developed on granular activated carbon particle electrodes by incorporating nano α-Fe₂O₃. The influences of operating parameters such as current, influent concentration, and hydraulic retention time (HRT) on denitrification are investigated as well as the mechanisms of SAHB-improved denitrification. The results indicate that SAHB achieves a high autotrophic denitrification rate of 4.20 mg TN/L/h under 40 mA current, 100 mg/L nitrate, and 26 h HRT, outperforming the naturally formed biofilm by 23.53%. SAHB contains more microorganisms than naturally formed biofilms, revealing that nano α-Fe₂O₃ is conducive to trapping free microorganisms on particle electrodes. SAHB enriches denitrification-related genes and increases corresponding enzyme activities (particularly nitrite reductase). According to metagenomic sequencing, nano α-Fe₂O₃ addition at a current level of 40 mA appears to favor denitrification via the nirS-dependent pathway probably due to its high affinity for cytochrome C. Metagenomic binning reveals that the microorganism associated with Zoogloeaceae (Rhodocyclales order) in SAHB may play a pivotal role in hydrogen autotrophic denitrification. This work develops a novel denitrification system, furthering our understanding of hydrogen autotrophic denitrification in 3D-BERs.